Enhanced Thermoelectric Properties of Polycrystalline Snse Via Lacl3 Doping

materials

Article Enhanced Thermoelectric Properties of Polycrystalline SnSe via LaCl3 Doping

Fu Li 1,2, Wenting Wang 1, Zhen-Hua Ge 1,3,*, Zhuanghao Zheng 1, Jingting Luo 1, Ping Fan 1 and Bo Li 3

1 Shenzhen Key Laboratory of Advanced Thin Films and Applications, College of Physics and Energy, Shenzhen University, Shenzhen 518060, China; [email protected] (F.L.); [email protected] (W.W.); [email protected] (Z.Z.); [email protected] (J.L.); [email protected] (P.F.) 2 Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China 3 Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China; [email protected] * Correspondence: [email protected]; Tel.: +86-0755-2690-5882

Received: 12 January 2018; Accepted: 26 January 2018; Published: 28 January 2018

Abstract: LaCl3 doped polycrystalline SnSe was synthesized by combining mechanical alloying (MA) process with spark plasma sintering (SPS). It is found that the electrical conductivity is enhanced after doping due to the increased carrier concentration and carrier mobility, resulting in optimization of the power factor at 750 K combing with a large Seebeck coefﬁcient over 300 Mvk−1. Meanwhile, all the samples exhibit lower thermal conductivity below 1.0 W/mK in the whole measured temperature. The lattice thermal conductivity for the doped samples was reduced, which effectively suppressed the increscent of the total thermal conductivity because of the improved electrical conductivity. As a result, a ZT value of 0.55 has been achieved for the composition of SnSe-1.0 wt % LaCl3 at 750 K, which is nearly four times higher than the undoped one and reveals that rare earth element is an effective dopant for optimization of the thermoelectric properties of SnSe.

Keywords: thermoelectric material; SnSe; electrical conductivity; thermal conductivity

1. Introduction Thermoelectric (TE) devices can convert waste heat directly and reversibly into electrical energy, which offers a prospect to provide a green and reliable source of energy because they have advantages of no moving parts, no noise, no vibration, and no emission of any harmful gases [1]. The energy conversion efficiency of a TE device is mainly determined by the figure of merit (ZT) for a given material, as ZT = S2σT/κ, where S, σ, κ, and T are Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively. Therefore, to break the bottleneck of the low energy conversion efficiency of TE devices that restricts their widespread application, great effort has been devoted to boosting the performances of the materials by enhancing the ZT values via improving the power factor (S2σ) and reducing the thermal conductivity in the past decades. Up to now, the ZT values have improved over 1.5, and even exceeds 2.0 for the conventional TE materials of PbTe and Bi2Te3 based compounds [2–4]. Meanwhile, considering the cost and environmental friendliness of the materials in further application, some competitive compounds [5–7] with non-toxic and inexpensive elements have been sought and found, which has also become an important research point in thermoelectric field in recent years. Tin selenide (SnSe) with an environmentally friendly composition is recently considered as a very promising TE material because of the high TE property of its single crystal [7–9]. A large ZT value over 2.5 was achieved along the b direction by combining the anisotropic electrical transport property and the extremely low thermal conductivity caused by the typical layered crystal structure [7]. By further

Materials 2018, 11, 203; doi:10.3390/ma11020203 www.mdpi.com/journal/materials Materials 2018, 11, 203 2 of 10

hole doping, a record high device dimensionless figure of merit ZTdev of 1.34 can be obtained, making it as a robust TE candidate for energy conversion applications [9,10]. However, the layered crystal structure also causes a poor mechanical property of the single crystal, since it can cleave easily along the (001) plane. Moreover, the harsh production conditions are still a disadvantage for practical application. Thereby, more attention has directed to polycrystalline SnSe in recent studies [11–13]. However, it is found that polycrystalline SnSe is much more difficult to yield a high ZT value than its single crystal form due to the higher lattice thermal conductivity and lower electrical transport property originating from the low intrinsic carrier concentration. For the sake of improving TE performance of polycrystalline SnSe, the study of element substitution at Sn or Se ions is a very important route, because the doping elements always lower the Fermi level to activate multiple valence bands together with introducing point defects and nanoscale precipitates to increase phonon scattering [14]. Thus, the carrier concentration can be optimized while the lattice thermal conductivity can be reduced, resulting in a great enhancement of the ZT value. In fact, the doping effect on SnSe has been extensively investigated in the past work and enhanced TE properties have been achieved by doping with various elements, such as Ag, Na, K, Ag, Pb, Ca, Cu, Br, I, and Ti [12,13,15–20]. For instance, a maximum ZT value of 1.2 was realized for the K and Na codoped SnSe at 773 K. Nevertheless, systematic studies on polycrystalline SnSe doped with rare earth elements with insight into their transport mechanisms are rather scarce so far [21]. In order to develop a useful and comprehensive guide to optimize their TE properties, more studies are needed. Actually, the TE properties of many other compounds have improved by doping rare earth elements due to the optimized carrier concentration and the depressed lattice thermal conductivity [22,23]. In this study, LaCl3 doped SnSe has been synthesized by combining mechanical alloying (MA) process with spark plasma sintering (SPS), which is as a simple and cost-effective approach used for fabrication of high-performance TE materials in recent years [24–26]. Considering the anisotropic TE transport properties along each crystallographic direction, the electrical and thermal transport properties were investigated along the same direction perpendicular to the SPS pressing direction. It has been found that the electrical properties have been improved and the lattice thermal conductivity can be reduced by doping LaCl3, inducing a synergically optimized final TE performance.

2. Materials and Methods

Polycrystalline SnSe-xwt% LaCl3 (x = 0.0, 0.5 and 1.0) samples were synthesized by combing mechanical alloying (MA) and spark plasma sintering (SPS), which is similar to our previous report [18,25]. High-purity starting raw materials Sn (99.999%) and Se (99.999%) with stoichiometric ratio were subjected to MA in argon (>99.5%) atmosphere. After milling for 8 h at 425 rpm in a planetary ball mill, the powders were mixed with LaCl3 and densified by SPS at 773 K for 5 min in graphite mold with Φ10 mm. The pressure is 50 MPa with uniaxial during the sintering. Series columnar- shaped specimens with dimensions of Φ10 × 8 mm were finally obtained, which were then cut and grinded into special shapes to measure their electrical conductivity, Seebeck coefficient, and thermal conductivity. The densities of the samples were tested by the Archimedes method. The phase structures were investigated by X-ray diffraction (XRD) with a D/max-RB diffractometer (Rigaku, Tokyo, Japan) using CuKα radiation via continuous scanning at a scanning rate of 6 0/min. The microscopic morphology of all the samples were identified with the help of scanning electron microscopy (Zeiss, supra 55 Sapphire, Munich, Germany). The composition of the sample and the distribution of the elements were observed by an energy dispersive X-ray spectrometer (EDS) attached to SEM. Using a Seebeck coefficient/electric resistance measuring system (ZEM-3, Ulvac-Riko, Yokohama, Japan), the Seebeck coefficient and electrical resistance data were recorded concurrently in a temperature range of 300 to 750 K. The Hall coefficients at room temperatures were measured using the van der Pauw Materials 2018, 11, 203 3 of 10 technique under a reversible magneticMaterials field 2018 of, 11 ,0.80 x FOR T PEER (8400 REVIEW Series, Model 8404, Lake Shore, 3 of 9 Watertown, IL, USA). The thermal diffusivity (D) was measured in the thickness direction of a square-shaped sample of 6 × 6 mm2 andandequation 1~2 1~2 mm mm к =in inDC thickness thicknesspd; where by by Cusinp using is gthe laser laser specific flash flash heatdiffusivity diffusivity capacity method method obtained (LFA467, from literature Netzsch,. I Selb,n the Germany).literature, the Cp was measured by using a simultaneous thermal analyzer (STA 447, Netzsch, Selb, Germany) (LFA467, Netzsch, Selb, Germany). TheThe thermal thermal conductivity conductivity ( (к) ) waswas calculatedcalculated accordingaccording toto thethe equation = DCpd; where Cp is the specific[7] and d heat is the capacity density. obtained from literature. In the literature, the C was measured by using a p simultaneousThe combined thermal uncertainty analyzer (STAof the 447, experimental Netzsch, Selb, determination Germany) of [7 ]ZT and is daboutis the 15~20% density. and caused by fiveThe respective combined measurements uncertainty of including the experimental electrical determination resistivity, Seebeck of ZT coefficient, is about 15~20% thermal and diffusion caused bycoefficient, five respective thermal measurements capacity, and including density, at electrical 3–4% for resistivity, each. Seebeck coefficient, thermal diffusion coefficient,Nomenclature: thermal capacity, S, Seebeck and coefficient; density, at 3–4%σ, electrical for each. conductivity; κ, thermal conductivity; κc, carrier thermal conductivity; κL, lattice thermal conductivity; T, absolute temperature; ZT, the figure Nomenclature: S, Seebeck coefficient; σ, electrical conductivity; κ, thermal conductivity; κc, carrier of merit; S2σ, power factor; SPS, spark plasma sintering; MA, mechanical alloying, XRD, X-ray thermal conductivity; κL, lattice thermal conductivity; T, absolute temperature; ZT, the figure of merit; diffraction; EDS, energy dispersive X-ray spectrometer; SEM, scanning electron microscopy; D, S2σ, power factor; SPS, spark plasma sintering; MA, mechanical alloying, XRD, X-ray diffraction; thermal diffusivity; Cp, specific heat; d, density. EDS, energy dispersive X-ray spectrometer; SEM, scanning electron microscopy; D, thermal diffusivity; C , specific heat; d, density. 3.p Results and Discussion

3. ResultsThe XRD and patterns Discussion of all the samples SnSe-xwt% LaCl3 (x = 0.0, 0.5 and 1.0) are shown in Figure 1. As compared with JCPDS data of PDF#48-1224, the patterns of all the compositions can be indexed The XRD patterns of all the samples SnSe-xwt % LaCl3 (x = 0.0, 0.5 and 1.0) are shown in Figure1. Asby compared SnSe characteristic with JCPDS peaks data with of PDF#48-1224, layered orthorhombic the patterns crystal of all the structure compositions and no can second be indexed phase by is SnSeobserved characteristic within the peaks detection with layered limits of orthorhombic the measurement. crystal structureHowever, and a slightly no second stronger phase intensity is observed of withinpeak (400) the detectionwith a weaker limits intensity of the measurement. of peak (100) However,compared a to slightly the sta strongerndard diffraction intensity ofpeaks peak can (400) be withobserved a weaker for all intensity the samples, of peak indicating (100) compared the existence to the of standard anisotropy diffraction and the peakspreferred can beorientations observed foroccurr alling the along samples, the indicating(400) plane. the To existenceconfirm the of anisotropypreferred orientation, and the preferred the preferential orientations factor occurring of all the alongsamples the was (400) calcula plane.ted To using conﬁrm the theLotgoering preferred method orientation, [25,27], the where preferential the bc orientation factor of all degree the samples for the was(h00) calculated crystal planes using of the the Lotgoering bulk sample method is termed [25,27 ], as whereF(h00). When the bc orientation the a-axes of degree crystal for grains the (h are00) completely aligned along the pressing direction, the calculated value of F is 1. In the present study, crystal planes of the bulk sample is termed as F(h00). When the a-axes of crystal grains are completely alignedthe calculated along theF(h00 pressing) values direction,are around the 0.34 calculated for all th valuee SnSe of-xFwt%is 1. LaCl In the3 (x present = 0.0, 0.5 study, and the 1.0) calculated samples, which reveals that axial the plane is slightly preferentially oriented in the bc-plane. This is reasonable F(h00) values are around 0.34 for all the SnSe-xwt % LaCl3 (x = 0.0, 0.5 and 1.0) samples, which reveals thatbecause axial SnSe the planecrystallized is slightly with preferentially layered structure oriented can ineasily the bcexhibit-plane. preferential This is reasonable orientation because following SnSe crystallizedthe uniaxial withdensification layered structureprocess, a cannd anisotropy easily exhibit has preferentialbeen widely orientation observed in following previous thestudies uniaxial [11– densiﬁcation13,28]. process, and anisotropy has been widely observed in previous studies [11–13,28].

Figure 1.1. XRDXRD patternspatterns ofof allall thethe LaClLaCl33 doped SnSe samples.samples.

SEM morphologies of the fresh fracture surfaces for the samples SnSe-0.5wt% LaCl3 and SnSe- SEM morphologies of the fresh fracture surfaces for the samples SnSe-0.5 wt % LaCl3 and 1.0wt% LaCl3 are presented in Figure 2a,b, respectively. The microstructure of the sample without SnSe-1.0 wt % LaCl3 are presented in Figure2a,b, respectively. The microstructure of the sample LaCl3 was also given in the inset of Figure 2a. It is seen that the undoped sample, with a relative high without LaCl3 was also given in the inset of Figure2a. It is seen that the undoped sample, with a relative density over 95% (the theoretical density of SnSe is 6.19 g/cm3), shows a dense morphology. The grain high density over 95% (the theoretical density of SnSe is 6.19 g/cm3), shows a dense morphology. is plate-shaped with size of 1~2 µm in diameter and 50–100 nm in thickness. However, after doping, The grain is plate-shaped with size of 1~2 µm in diameter and 50–100 nm in thickness. However, after the grain size is reduced to 500 nm ~1 µm and decreased as the doping content increased. The plate shaped grains tended to become equiaxed grain due to the reduction of the grain size. It is clear that

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µ Materialsdoping, 2018 the, 11 grain, x FOR size PEER is REVIEW reduced to 500 nm~1 m and decreased as the doping content increased.4 of 9 The plate shaped grains tended to become equiaxed grain due to the reduction of the grain size. It is theclear reduced that the grain reduced size grainis mainly size iscaused mainly by caused the introduced by the introduced foreign atoms foreign since atoms they since would they prevent would theprevent diffusion the diffusion of atoms of during atoms the during sintering the sintering and suppress and suppress the growth the growthof grains. of Nevertheless, grains. Nevertheless, such a finesuch-grained a ﬁne-grained microstructure microstructure is desirable is desirable for TE materials, for TE materials, whose thermal whose conductivity thermal conductivity can be reduced can be byreduced their enhanced by their enhanced phonon phononscattering scattering due to the due increased to the increased grain boundaries. grain boundaries. In addition, In addition, it is also it is foundalso found that the that porosity the porosity increased increased when when the doped the doped content content raised raised to 1%, to which 1%, which is consistent is consistent with with the reducedthe reduced density. density. This This might might be caused be caused by some by some volatilization volatilization of Cl. of Cl.Moreover, Moreover, the thedistributions distributions of theofthe elements elements Sn, Sn, Se, Se, La La,, and and Cl Cl on on the the polished polished surface surface of of the the representative samplesample SnSe-0.5SnSe-0.5 wt% % LaClLaCl33 werewere observed observed by by EDS, EDS, which which suggests suggests that that all all the the elements elements are are distributed distributed homogeneous homogeneously.ly.

Figure 2. (a) and (b) SEM micrographs of the fresh fracture surfaces for the doped bulks SnSe-0.5wt% Figure 2. (a) and (b) SEM micrographs of the fresh fracture surfaces for the doped bulks SnSe-0.5 wt % LaCl3 and SnSe-1.0wt% LaCl3, respectively. The morphology of the sample without LaCl3 was given LaCl3 and SnSe-1.0wt % LaCl3, respectively. The morphology of the sample without LaCl3 was given inset of Figure 2a. inset of Figure2a.

Figure 3 shows the electrical transport properties dependence with the measured temperature Figure3 shows the electrical transport properties dependence with the measured temperature for all the samples SnSe-xwt% LaCl3 (x = 0.0, 0.5 and 1.0), which are measured along the direction perpendicularfor all the samples to the SnSe- SPS pressingxwt % LaCl direction3 (x = since 0.0, 0.5 the and samples 1.0), whichshow existence are measured of anisotropy. along the As direction shown inperpendicular Figure 3a, the to electrical the SPS pressing conductivities direction of sinceall the the samples samples increase show existence with increasing of anisotropy. temperature As shown in thein Figure whole3 measureda, the electrical temperature, conductivities indicating of all a typical the samples semiconducting increase with behavior increasing with temperaturenondegenerate in characteristicthe whole measureds. It is also temperature, found that indicating the electrical a typical conductivity semiconducting for the undoped behavior sample with nondegenerate is lower than thecharacteristics. doped ones,It which is also shows found about that the 8.53 electrical × 10−4 Scm conductivity−1 at room temperature for the undoped which sample then increases is lower than nearly the −4 −1 fourdoped orders ones, of which magnitude shows at about 750 K 8.53, reaching× 10 Scm6.38 Scmat−1 room, which temperature is similar to which the previous then increases report nearly [18]. four orders of magnitude at 750 K, reaching 6.38 Scm−1, which is similar to the previous report [18]. After LaCl3 doping, the value has improved to about 0.02 Scm−1 at room temperature, and then −1 sharplyAfter LaCl increased3 doping, in range the value of 650 has to 750 improved K. An electrical to about conductivity 0.02 Scm atof room15.55 Scm temperature,−1 at 750 K andis finally then −1 obtained,sharply increased which is in two range times of higher 650 to than 750 K. that An of electrical the undoped conductivity samples of at 15.55 the same Scm temperature.at 750 K is Additionally,ﬁnally obtained, similar which electrical is two times conductivities higher than in thatthe whole of the undopedtemperature samples range at were the same obtained temperature. for the Additionally, similar electrical conductivities in the whole temperature range were obtained for the samples doped with 0.5% and 1.0% LaCl3. No obvious differences can be observed with increasing thesamples doped doped content. with The 0.5% electrical and 1.0% conductivity LaCl3. No obviouscan be expressed differences as canσ = benµe observed, where e withis the increasing unit charge, the ndoped is the content.carrier concentration The electrical, and conductivity µ is the carrier can be mobility, expressed respectively. as σ = nµe Thus,, where thee isenhanced the unit electrical charge, n conductivityis the carrier after concentration, doping should and µbeis ascribed the carrier to the mobility, increased respectively. carrier concentration Thus, the enhanced or the mobility electrical or bothconductivity of them. afterThe Hall doping carrier should concentration be ascribed and to the Hall increased mobility carrier at room concentration temperature or for the the mobility samples or both of them. The Hall carrier concentration and Hall mobility at room temperature for the samples SnSe and SnSe-0.5 wt% LaCl3 have been measured. The results indicate that the value of the carrier concentratSnSe and SnSe-0.5ion and carrier wt % LaCl mobility3 have for been the measured. undoped Thesample results are indicate7.40 × 10 that14 cm the−3 and value 7.23 of thecm2 carrierV−1s−1, 14 −3 2 −1 −1 respectively.concentration Thus, and carrierthe lower mobility carrier for concentration the undoped and sample carrier are 7.40mobility× 10 shouldcm beand the 7.23 reason cm Vfor sthe , lowerrespectively. electrical Thus, conductivity the lower of the carrier undoped concentration sample, which and carrier is in agreement mobility with should the beprevious the reason reports for the lower electrical conductivity of the undoped sample, which is in agreement with the previous [11–13]. However, for the doped sample with 0.5 wt% LaCl3, the carrier concentration has increased toreports 1.53 ×[ 1011–1613 cm].− However,3, which is fornearly the dopedone order sample of magnitude with 0.5wt higher % LaCl than3, thethat carrier of the concentrationundoped sample. has 16 −3 increased to 1.53 × 10 cm , which is nearly one order of2 magnitude−1 −1 higher than that of the undoped Meanwhile, the carrier mobility also increases to 12.3 cm V s for the sample with 0.5 wt% LaCl3. 2 −1 −1 Thissample. indicates Meanwhile, that the thetwo carrier effects mobility of improvement also increases of both to carrier 12.3 cm concentrationV s for and the samplecarrier mobility with 0.5 wt % LaCl . This indicates that the two effects of improvement of both carrier concentration and lead to the 3enhancement in electrical conductivity, suggesting that LaCl3 is an effective dopant in optimizingcarrier mobility the electrical lead to the transport enhancement properties in electrical of SnSe. conductivity, The LaCl3 doping suggesting in SnSe that matrix LaCl3 is ana strong effective n- type doping. The La substituting Sn and the Cl substituting Se both increase the electron concentrations in p-type SnSe, but the electrical conductivity of the doped sample is increased. This may be due to the low carrier concentration of the undoped sample. That is, the increasing of electron concentration increased the total carrier concentration as the minority carrier. Furthermore, just a

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dopant in optimizing the electrical transport properties of SnSe. The LaCl3 doping in SnSe matrix is a strong n-type doping. The La substituting Sn and the Cl substituting Se both increase the electron Materialsconcentrations 2018, 11, x FOR in p-type PEER SnSe,REVIEW but the electrical conductivity of the doped sample is increased. This 5 of 9 may be due to the low carrier concentration of the undoped sample. That is, the increasing of electron littleconcentration bit of LaCl increased3 should the be total doped carrier in concentration the SnSe matrix, as the minority because carrier. of the Furthermore, decreased just grain a little size and increasedbit of LaCl pores3 should with be the doped increasing in the SnSeLaCl matrix,3 contents because observed of the in decreased SEM images. grain size and increased pores with the increasing LaCl3 contents observed in SEM images.

FigureFigure 3. 3. ElectricalElectrical transport propertiesproperties as as a functiona function of temperature of temperature for all for the all samples the samples SnSe-x wtSnSe % -xwt%

LaClLaCl3 (3x( x= =0.0, 0.0, 0.5, 0.5, and and 1.0). 1.0). ( a) electricalelectrical conductivity; conductivity (b; )( Seebeckb) Seebeck coefﬁcient; coefficient (c) power; (c) power factor. factor.

FigureFigure 3b3b presents presents the Seebeck coefﬁcientscoefficients of of all all the the samples samples as as a a function function of of temperature. temperature. The positiveThe positive values values of the of theSeebeck Seebeck coefficient coefﬁcient for for the undopedundoped sample sample demonstrate demonstrate a p-type a p- conductiontype conduction byby holes holes as as the the dominate dominate carriers. ItIt is is also also noted noted that, that although, although it is it a nis-type a n- dopingtype doping when LaClwhen3 isLaCl3 is doped in SnSe, all the values in the whole temperature range are positive, which is also consistent with the measured Hall coefficient with positive sign. Therefore, this means that a small amount of LaCl3 substitution cannot change its conductive mechanism. This result is different from the dopant of BiCl3 in the previous report [29], in which a small amount of dopant can change the conductive mechanism of SnSe form p-type to n-type. La3+ doping and Bi3+ doping are all the electron doping (n type doping) for SnSe matrix, but the SnSe is an intrinsic p-type material due to the Sn vacancies. In this work, the Sn was reduced and we assumed that La3+ will stay in the position of Sn for introducing electrons to make the SnSe n-type. The results showed that the LaCl3 could not effectively replace Sn and introduce enough electrons to make the n-type change of SnSe compared to BiCl3, and the reduced Sn will increase Sn vacancies to introduce holes in the matrix and retain the SnSe matrix p-

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[7]. By further hole doping, a record high device dimensionless figure of merit ZTdev of 1.34 can be obtained, making it as a robust TE candidate for energy conversion applications [9,10]. However, the layered crystal structure also causes a poor mechanical property of the single crystal, since it can cleave easily along the (001) plane. Moreover, the harsh production conditions are still a disadvantage for practical application. Thereby, more attention has directed to polycrystalline SnSe in recent studies [11–13]. However, it is found that polycrystalline SnSe is much more difficult to yield a high ZT value than its single crystal form due to the higher lattice thermal conductivity and lower electrical transport property originating from the low intrinsic carrier concentration. For the sake of improving TE performance of polycrystalline SnSe, the study of element substitution at Sn or Se ions is a very important route, because the doping elements always lower the Fermi level to activate multiple valence bands together with introducing point defects and nanoscale precipitates to increase phonon scattering [14]. Thus, the carrier concentration can be optimized while the lattice thermal conductivity can be reduced, resulting in a great enhancement of the ZT value. In Materialsfact, the2018 doping, 11, 203 effect on SnSe has been extensively investigated in the past work and enhanced6 of 10TE properties have been achieved by doping with various elements, such as Ag, Na, K, Ag, Pb, Ca, Cu, dopedBr, I, inand SnSe, Ti [12,13,15–20]. all the values inFor the instance, whole temperature a maximum range ZT value are positive, of 1.2 was which realized is also for consistent the K and with Na codoped SnSe at 773 K. Nevertheless, systematic studies on polycrystalline SnSe doped with rare the measured Hall coefficient with positive sign. Therefore, this means that a small amount of LaCl3 earth elements with insight into their transport mechanisms are rather scarce so far [21]. In order to substitution cannot change its conductive mechanism. This result is different from the dopant of BiCl3 indevelop the previous a useful report and [29 comprehensive], in which a small guide amount to optimi of dopantze their can TE change properties, the conductive more studies mechanism are needed. of SnSeActually, form p the-type TE to propertiesn-type. La of3+ madopingny other and compounds Bi3+ doping have are all improved the electron by dopingdoping ( nraretype earth doping) elements for SnSedue matrix, to the optimized but the SnSe carrier is an intrinsicconcentrationp-type and material the de duepressed to the lattice Sn vacancies. thermal In conductivity this work, the [22,23]. Sn was In reducedthis study, and LaCl we assumed3 doped thatSnSe La has3+ willbeen stay synthesized in the position by combining of Sn for introducingmechanical electronsalloying (MA) to make process the with spark plasma sintering (SPS), which is as a simple and cost-effective approach used for SnSe n-type. The results showed that the LaCl3 could not effectively replace Sn and introduce enough fabrication of high-performance TE materials in recent years [24–26]. Considering the anisotropic TE electrons to make the n-type change of SnSe compared to BiCl3, and the reduced Sn will increase Sn vacanciestransport to properties introduce along holes ineach the crystallographic matrix and retain direction, the SnSe the matrix electricalp-type. and However, thermal as transport shown inproperties Figure3, high were absolute investigated Seebeck along coefficients the same directio over 150n perpendicularµVK−1 are obtained to the SPS in the pressing whole direction. measured It temperaturehas been found range that for the all electrical the samples. properties For the have undoped been improved sample, and the Seebeckthe lattice coefficient thermal conductivity decreased ascan the be temperature reduced by increased,doping LaCl which3, inducing is 318 µaVK synergically−1 at room optimized temperature, final declining TE performance. to 150 µ VK−1 at 750 K, this trend is agreement with the previous report [30]. Nevertheless, the values for all the doped 2. Materials and Methods samples have similar strong temperature dependence, which firstly increases and then decreases until 500 K,Polycrystalline and further improves SnSe-x aswt% the LaCl temperature3 (x = 0.0, increases 0.5 and up 1.0) to samples 700 K. The were Seebeck synthesized coefficient by is combing nearly 400mechanicalµVK−1 at alloying 700 K and (MA) 350 andµVK spark−1 at plasma 750 K, sintering which is (SPS), two times which larger is similar than theto our undoped previous sample report although[18,25]. High-purity the electrical starting conductivity raw materials has enhanced Sn (99.999%) as shown and in Se Figure (99.999%)3a. The with Seebeck stoichiometric coefficient ratio is relatedwere subjected to the scattering to MA in factor argon according (>99.5%) toatmosphere. the equation AfterS = millingκB/e( γfor− 8ln h( nat/ 425N0)) rpm, where in a nplanetaryand γ areball carrier mill, concentrationthe powders were and scatteringmixed with factor, LaCl respectively3 and densified [25]. by The SPS substitution at 773 K for of 5 LaCl min3 canin graphite cause structuralmold with distortions Φ10 mm. due The to pressure the difference is 50 of MPa the ionicwith radiusuniaxial with during SnSe. the Thus, sintering. the optical Series phonon columnar- may beshaped scattered, specimens causing anwith improvement dimensionsof of scattering Φ10 × 8 mm parameter were finally and the obtained, Seebeck coefficient.which were Combing then cut theand Seebeckgrinded coefficient into special (S) shapes and the to electrical measure conductivity their electrical (σ), conductivity, the power factor Seebeck (PF = coefficient,S2σ) dependence and thermal with temperatureconductivity. was calculated as shown in Figure3c. All the values increase with increasing temperature in the wholeThe densities measured of the temperature samples were range. tested The power by the factor Archimedes below 600 method. K for allThe the phase samples structures are similar were andinvestigated lower than by 25 X-rayµWm −diffraction1K−2 because (XRD) of the with poor a D/max-RB electrical conductivity. diffractometer However, (Rigaku, benefiting Tokyo, Japan) from theusing enhanced CuKα electrical radiation conductivity via continuous and Seebeck scanning coefficient at a scanning over 600 rate K, the of power 6 0/min. factor The for microscopic the doped samplemorphology has obviously of all the been samples enhanced. were identified A maximum with valuethe help of ~200of scanningµWm− 1electronK−2 at 750microscopy K is obtained (Zeiss, forsupra the LaCl55 Sapphire,3 doped sampleMunich, of German SnSe-xwty). %The LaCl composition3 with x = of 0.5 the and sample 1.0, which and the is nearly distribution an order of ofthe magnitudeelements were larger observed than that by of an the energy undoped dispersive sample. X-ray spectrometer (EDS) attached to SEM. Using a SeebeckFigure coefficient/electric4a, b shows the thermal resistance transport measuring properties system as a (ZEM-3, function Ulvac-Riko, of temperature Yokohama, for all the Japan), samples the 2 SnSe-Seebeckx wt coefficient % LaCl3 (x and= 0.0, electrical 0.5, and resistance 1.0). All the data thermal were diffusivitiesrecorded concurrently are in range in of a 0.7temperature to 0.15 mm range/s andof 300 decrease to 750 gradually K. The Hall as the coefficients temperature at room increases. temperatures The changing were trend measured of the thermalusing the conductivity van der Pauw is consistenttechnique with under that ofa thereversible thermal magnetic diffusivity. field For theof undoped0.80 T (8400 SnSe, Series, the thermal Model conductivity 8404, Lake at roomShore, temperatureWatertown, is IL, 0.94 USA). W/mK. The With thermal rising diffusivity temperature, (D the) was value measured reduces in to 0.24the W/mKthickness at 750direction K, which of a issquare-shaped very close to the sample reported of 6 × values 6 mm of2 and the 1~2 undoped mm in polycrystallinethickness by usin samplesg laser [flash13]. Thediffusivity total thermal method conductivity(LFA467, Netzsch, is usually Selb, summed Germany). by electronic The thermal thermal conductivity conductivity ( (кe ) andwas latticecalculated thermal according conductivity to the ( ), where is the phonon contribution and is the electronic contribution which is related to the l l e electrical conductivity according to the Wiedemann–Franz–Lorenz relation, that is e = σLT, where L is Lorenz number, σ is the electrical conductivity, and T is the absolute temperature [31]. In the present study, the e holds less than 7% of the total thermal conductivity, revealing that the heat transport of SnSe is predominated by phonon. After doping with 0.5 wt % LaCl3, the thermal conductivity slightly increases in the whole measured temperature, which decreases with increasing temperature from 0.98 W/mK around room temperature to 0.47 W/mK at 750 K. However, when the doped content increases to 1.0%, the total thermal conductivity reduces, especially in range of 300 to 650 K. Since the electrical conductivity of the undoped sample is lower than the doped ones as shown in Figure3a, the slightly increased thermal conductivity for the sample with a lower doping content of 0.5% is mainly caused by the improved electronic thermal conductivity. With further increases of the doped content, phonon scattering would be enhanced due to the increased crystal defects introduced by the doping. Thus, the lattice thermal conductivity would further decrease, resulting in a reduction Materials 2018, 11, 203 7 of 10

of the total thermal conductivity for the SnSe-1.0 wt % LaCl3 sample. Additionally, as discussed above, the grain size is reduced obviously after doping, which would lead to a higher grain boundary concentration for the doped samples. Therefore, phonons would suffer more scattering along grain Materialsboundaries, 2018, 11 which, x FOR would PEER REVIEW induce a lower lattice thermal conductivity and cause a lower total thermal 7 of 9 conductivity for the sample doped with 1.0 wt % LaCl3 than the undoped one.

FigureFigure 4. 4. ThermalThermal transport propertiespropertie ass as a functiona function of temperature of temperature for all for the all samples the samples SnSe-x wtSnSe % -xwt%

LaClLaCl3 (3x( x= =0.0, 0.0, 0.5, 0.5, and and 1.0). 1.0). ( a) Thermal diffusivity; diffusivity (b; )(b thermal) thermal conductivity. conductivity.

TheThe ZTZT valuevalue as as shown shown in FigureFigure5 5was was calculated calculated by combiningby combining the electrical the electrical transport transport property property andand thermal thermal conductivity. conductivity. This showsshows that that the theZT ZTvalues values for for all theall samplesthe samples increase increase with increasingwith increasing temperature.temperature. Because Because of of the the lower lower electrical electrical transport transport property andand higher higher thermal thermal conductivity, conductivity, the maximumthe maximum ZT valueZT value for forthe the undoped undoped sample sample is onlyonly 0.15. 0.15. For For all all the the doped doped samples, samples, the values the values are are similarsimilar to to those those of of the undopedundoped ones ones in in the the temperature temperature range range of 300 of to 300 600 to K. 600 However, K. However, attributed attribut to ed to the the enhanced enhanced electrical electrical conductivity conductivity and reduced and reduced lattice thermal lattice conductivity thermal co withnductivity rising the measuredwith rising the temperature after doping, the ZT value is enhanced and a maximum value of 0.55 is obtained for the measured temperature after doping, the ZT value is enhanced and a maximum value of 0.55 is composition of SnSe-1.0 wt % LaCl3 at 750 K, which is nearly four times higher than the undoped one. obtained for the composition of SnSe-1.0wt% LaCl3 at 750 K, which is nearly four times higher than This result suggests that rare earth elements are an effective dopant for improving the TE properties theof undoped SnSe. one. This result suggests that rare earth elements are an effective dopant for improving the TE properties of SnSe.

Figure 5. ZT values as a function of temperature for all the samples SnSe-xwt% LaCl3 (x = 0.0, 0.5, and 1.0).

Materials 2018, 11, x FOR PEER REVIEW 7 of 9

Figure 4. Thermal transport properties as a function of temperature for all the samples SnSe-xwt%

LaCl3 (x = 0.0, 0.5, and 1.0). (a) Thermal diffusivity; (b) thermal conductivity.

The ZT value as shown in Figure 5 was calculated by combining the electrical transport property and thermal conductivity. This shows that the ZT values for all the samples increase with increasing temperature. Because of the lower electrical transport property and higher thermal conductivity, the maximum ZT value for the undoped sample is only 0.15. For all the doped samples, the values are similar to those of the undoped ones in the temperature range of 300 to 600 K. However, attributed to the enhanced electrical conductivity and reduced lattice thermal conductivity with rising the measured temperature after doping, the ZT value is enhanced and a maximum value of 0.55 is obtained for the composition of SnSe-1.0wt% LaCl3 at 750 K, which is nearly four times higher than the undoped one. This result suggests that rare earth elements are an effective dopant for improving Materials 2018, 11, 203 8 of 10 the TE properties of SnSe.

Figure 5. ZT values as a function of temperature for all the samples SnSe-xwt% LaCl3 (x = 0.0, 0.5, and Figure 5. ZT values as a function of temperature for all the samples SnSe-x wt % LaCl3 (x = 0.0, 0.5, and 1.0). 1.0). 4. Conclusions

Polycrystalline SnSe-xwt % LaCl3 (x = 0.0, 0.5, and 1.0) was prepared by combing mechanical alloying (MA) process and spark plasma sintering (SPS). The phase structure and microstructure were observed, and their TE transport properties were investigated in the temperature range of 300 to 750 K. The results indicate that pure phase with a slightly preferred orientation grown along (400) can be obtained for all the samples and the grain size can be reduced after doping. Moreover, the electrical conductivity was improved to 15.55 Scm−1 at 750 K for the doped sample because of the increased carrier concentration and carrier mobility, which resulted in an optimization of the power factor combing with the moderate Seebeck coefﬁcient over 300 µVK−1. Finally, a ZT value of 0.55 has been obtained for the composition of SnSe-1.0 wt % LaCl3 at 750 K in combination with the reduced thermal conductivity below 1.0 Wm−1 K−1, which is nearly four times higher than the undoped one.

Acknowledgments: This work was supported by National Nature Science Foundation (grant no. 11764025, 51302140), Shenzhen Science and Technology Plan Project (no. JCYJ20160422102622085, JCYJ20150827165038323), as well as the Natural Science Foundation of SZU (no. 2016016). Author Contributions: Fu Li prepared the samples; Wenting Wang and Zhuanghao Zheng preformed thermoelectric properties measurements; Fu Li and Zhenhua Ge analyzed the data and wrote the paper; Zhenhua Ge, Bo Li, Jingting Luo, and Ping Fan contributed reagents/materials/analysis tools. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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